1. Field of the Invention
The present invention relates to a vehicle body frame of a motorcycle. Particularly, the present invention relates to a structure of a cross member for coupling a main frame having right and left members extending above an engine and others to a frame behind the main frame.
2. Description of the Related Art
Conventionally, in a vehicle body frame of a motorcycle, typically, a pivot frame extending downward and a rear frame extending rearward such that it is tilted in an upward direction are coupled to a rear end of a main frame extending rearward and obliquely downward from a head pipe. A front wheel is mounted to the head pipe via a front fork. A rear wheel is mounted to the pivot frame via a swing arm. The main frame, which receives a force applied by the front and rear wheels, is required to have a proper flexibility, i.e., a proper bending stiffness and a proper twisting stiffness.
Typically, an engine and a transmission are laid out below the main frame. The main frame is composed of a pair of right and left main frame members which are spaced apart from each other to provide a good balance between the bending stiffness and the twisting stiffness. For example, Japanese Laid-Open Patent Application Publication No. 2010-058763 discloses a motorcycle in which a pipe frame is composed of a pair of right and left pipes extending from a head pipe to a location near a rear end portion of a vehicle body, and a cross pipe coupling them together in a suitable location. The right and left pipe members are coupled together by a fuel tank bracket in a location rearward relative to substantially center portions thereof in a forward and rearward direction.
Rear arm brackets are welded to the right and left pipes of the pipe frame, respectively, such that the rear arm brackets extend downward from substantially center portions of the right and left pipes in the forward and rearward direction. The rear arm brackets support front ends of the rear arms (swing arms), respectively. Further, back-stays extend rearward and obliquely upward from portions of the right and left rear arm brackets, respectively. Rear ends of the back-stays are welded to rear ends of the pipe frame, respectively. That is, in this example, a portion of the pipe frame which is forward relative to the rear arm brackets is the main frame, and a portion of the pipe frame which is rearward relative to the main frame constitutes a rear frame along with the back-stays.
In recent years, to improve safety, to meet emission regulation, etc., devices incorporated into a motorcycle, for example, an ABS unit, a GPS unit, and a canister of an evaporative fuel, have been increasing in number. Therefore, attention has been paid to a space in which these devices are laid out. For example, Japanese Laid-Open Patent Application Publication No. 2009-292350 discloses a mounting structure for laying out an ABS unit between and slightly behind right and left rear arm brackets, in other words, in a forward portion of the rear portion of the vehicle body frame, in a motorcycle including a pipe frame similar to the pipe frame disclosed in the Patent Application stated above.
However, in the latter conventional example, pipes, stays, and others constituting a vehicle body frame, such as the right and left pipes of the pipe frame, the fuel tank bracket, the rear arm brackets, a cross pipe, and back stays, surround the ABS unit from above and from right and left sides. Therefore, it is very difficult to mount the ABS unit to the vehicle body.
To be specific, when the ABS unit is laid out in a space between the pipes and others surrounding the ABS unit, and bolts and nuts are tightened, interference with the surrounding pipes and others tends to occur. In addition, when a brake pipe is attached to the ABS unit mounted in this way, interference with the pipes and others is unavoidable. Such a problem associated with mounting work occurs in a case where devices other than the ABS unit, for example, a GPS unit and a canister, are mounted to the vehicle body frame.
The canister is preferably positioned closer to a fuel tank to reduce a length of hose used to introduce an evaporative fuel to the canister. Regarding this, in a conventional pipe work frame, the pipes and the like will become an obstruction. In the conventional example, for example, two pipes of the pipe frame extend through right and left sides of the vehicle body, and therefore, it is difficult to position the fuel tank closer to the canister below the fuel tank.
Note that pipe works of the frame considering the layout of the fuel tank and the devices may be implemented in theory. Typically, in the middle of development of the motorcycle, pipes and brackets are added to a portion of the vehicle body frame which is insufficient in stiffness. If these pipes and the like interfere with the fuel tank and the devices, all of the components must be laid out from the beginning.
In view of the above circumstances, an object of the present invention is to provide a vehicle body frame having a structure which can ensure a stiffness equal to or higher than that of pipe works while achieving space saving in a motorcycle, allows a fuel tank and devices to be laid out flexibly, and allows the fuel tank and the devices to be mounted to the vehicle body frame easily and efficiently.